A valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. In an open valve, fluid flows in a direction from higher pressure to lower pressure.
The simplest valve is simply a freely hinged flap which drops to obstruct the fluid flow in one direction, but is pushed open by flow in the opposite direction. This is called a check valve, as it prevents or “checks” the flow in one direction.
Valves have many uses, including controlling water for irrigation, industrial uses for controlling processes, residential uses such as on/off and pressure control to dish and clothes washers and taps in the home. Even aerosols have a tiny valve built in. Valves are also used in the military and transport applications.
Valves may be operated manually, either by a handle, lever, pedal or wheel. Valves may also be automatic, driven by changes in pressure, temperature, or flow. These changes may act upon a diaphragm or a piston which in turn activates the valve, examples of this type of valve found commonly are safety valves fitted to hot water systems or boilers.
More complex control systems using valves requiring automatic control based on an external input (i.e., regulating flow through a pipe to a changing set point) require an actuator. These valves requiring automatic control may also be termed process valves. An actuator will stroke the process valve depending on its input and set-up, allowing the valve to be positioned accurately, and allowing control over a variety of requirements.
Such an actuator may be operated using a set of main stage valves, operated by a pilot stage valve, which pilot stage valve operates based on received control signals.
There do exist “digital” pilot stage valves, operating between two states.
One such pilot stage valve for a digital main stage valve is described in U.S. Pat. No. 4,103,710.
In these pilot stage valves there is the question of how to handle a situation when power has been lost. In U.S. Pat. No. 4,103,710, the valve has a so-called fail safe behaviour at the loss of power, and therefore a pressure is provided to the main valve corresponding to a fully open pilot valve, which will put also the main valve in a fully open or a fully closed position.
This also means that the controlled process element, which is the above mentioned process valve main valve will end up being fully open or fully closed.
However, there are at times of interests to obtain a fail freeze behaviour and that is that the process valve should retain the same position it had before loss of power. This is not possible to obtain with the known digital pilot valve arrangements.
There is therefore a need for a pilot stage valve arrangement for a main stage valve, where the pilot valves of the pilot stage valve arrangement operate between an on and an off, which pilot valve arrangement has a fail freeze mode.